Water-resistant pouch type secondary cell

ABSTRACT

The present invention relates to a pouch type secondary cell with high water-resistance. The pouch type secondary cell comprises a positive electrode, a separation layer, and a negative electrode. Here, the sealing unit of the secondary cell includes steps so that the sealing unit has an outer side thinner than an inner side in thickness. The present invention is advantageous in that the manufacture process can be simplified, the water-resistance and sealing property of sealing portions can be further improved, and the manufacture costs can be reduced.

The present application claims priority to Korean Application No.10-2009-0044318 filed in Korea on May 21, 2009, the entire contents ofwhich is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a pouch type secondary cell with highwater-resistance.

2. Discussion of the Related Art

A secondary cell has a structure in which the electrode assembly of apositive electrode, a separation layer, and a negative electrode whichcan be charged or discharged is embedded in the casing of a laminatesheet, including a metal can or a resin layer and a metal layer, such asa cylinder type or an angular type, with an electrolyte impregnatedtherein. The electrode assembly can be classified depending on itsstructure. The electrode assembly can include, for example, a cylindertype (i.e., a take-up type) to Jelly-roll type electrode assembly, astack type electrode assembly in which a number of positive electrodesand negative electrodes cut in a specific size unit are sequentiallystacked with a separation layer interposed therebetween, a stack/foldingtype electrode assembly in which bi-cells or full cells, each havingpositive electrodes and negative electrodes of a specific unit stackedtherein with a separation layer interposed therebetween, are taken up,etc.

From among the cell structures, the cylinder type cell structure isadvantageous in that it is excellent in the structural stability, andthe cell structure using the laminate sheet casing are advantageous inthat it is light in weight and can be easily manufactured. Recently, theuse of the cell using the laminate sheet is increased suddenly inaccordance with a tendency toward the downsizing, light weight, andthinness of electrons devices and a need for a reduction in the weightof medium and large-sized cell packs. The cell using the laminate sheetis frequently called a pouch type secondary cell because of the shape ofa casing.

FIG. 1 is a dismantled perspective view which is pertinent to themanufacture of one secondary cell using a laminate sheet as a cellcasing (hereinafter referred to as a ‘pouch type secondary cell’).

Referring to FIG. 1, the pouch type secondary cell 100 is manufacturedby mounting an electrode assembly 300, formed of a positive electrode, aseparation layer, and a negative electrode, on a pouch type cell casing200 formed of a laminate sheet made of polymer resin and aluminum (Al)and then coupling electrode leads 410 and 420 to the cell casing 200with them exposed at the top of the cell casing 200.

The cell casing 200 includes an upper cover 230 and a lower casing 220having a reception unit 210 formed therein. The cell casing 200 has afolder type structure having a bottom integrated.

In the state in which the electrode assembly 300 is seated in thereception unit 210, the top surface 240 of the lower casing 220 and theouter circumferential surfaces 250 on both sides of the lower casing 220are adhered to the contact surface of the upper cover 230 and sealedtogether. Accordingly, after the cell is assembled, the top surface 240of the lower casing 220 and the outer circumferential surfaces 250 onboth sides of the lower casing 220 form a sealing unit.

Electrode tabs 310 and 320 protruded from the electrode assembly 300 areconnected to the respective electrode leads 410 and 420. Sealing films500 are connected to respective portions where the cell casing 200 andthe electrode leads 410 and 420 are connected to each other. The sealingfilms 500 function to prevent the leakage of an electrolyte, preventmoisture in air from infiltrating the cell, and guarantee the electricalinsulating property of the electrode leads 410 and 420.

FIG. 2 shows an example of another pouch type secondary cell and it is aperspective view showing the pouch type secondary cell in whichelectrode leads are respectively protruded from the top and bottom of acell casing.

The pouch type secondary cell 101 of FIG. 2 differs from the pouch typesecondary cell 100 of FIG. 1 in that the electrode leads 411 and 421 arerespectively disposed at the top and bottom of the cell casing and thecell casing is separated into a lower casing 221 and an upper casing231. Accordingly, the cell casing consists of the upper sealing unit241, the lower sealing unit 261, and the sealing units 251 and 271 onboth sides which are formed by thermally compressing the lower casing221 and the upper casing 231. A reception unit 211 can be formed only inthe upper casing 231 or the lower casing 221 or can be formed in boththe lower and upper casings 221 and 231.

FIG. 3 shows a process of forming the sealing unit of a laminate sheetwhich is commonly used as a cell casing in a pouch type secondary celland shows a cross section of the coupled laminate sheet.

Referring to FIG. 3, the laminate sheet 10 includes an external resinlayer 11 forming the outermost part, a metal layer 12 preventing thepenetration of materials, and an internal resin layer 13 performing asealing function.

The external resin layer 11 functions to protect the cell from theoutside and so requires the thickness versus an excellent tensionstrength, atmosphere corrosion resistance, etc. The external resin layer11 is commonly made of flexible nylon. The metal layer 12 functions toprevent air, moisture, etc. from being introduced into the cell and itis commonly made of aluminum (Al). The internal resin layer 13 isthermally compressed by heat and pressure applied in the state in whichan electrode assembly is built in the cell casing, thus providing asealing property. The internal resin layer 13 is commonly made of castpolypropylene (CPP).

The cell casing sheet 10 of the multi-layer laminate structure isconfigured to have the internal resin layers 13 facing each other in thesealing unit. The internal resin layers 13 are coupled together bythermal compression. In this case, the internal resin layer 13 isexposed externally at a portion where the laminate sheets are coupledtogether. Moisture can easily penetrate into the exposed internal resinlayer 13 because it is commonly made of polymer resin. The penetratedmoisture has bad problems in terms of cell safety, such as thatgenerates a side reaction within the cell, reduces the life span of thecell, oxidizes the metal layer 12 of the cell casing, weakens theadhesion strength of the sealing unit, and possibly leaks theelectrolyte. Further, if the cell is used for a long time, thepenetrated moisture reduces the life span and safety of the cell.Accordingly, various attempts to prevent the penetration of moisture andthe leakage of the electrolyte had been made.

For example, Japanese Unexamined Patent Application Publication No.2004-087239 discloses a laminate sheet in which an internal resin layeris coated with metal layers by shaping lateral portions where laminatefilms are adhered together through thermal pressurization andcompression so that the metal layers (i.e., shut-off metal layers) arebrought into contact with each other.

In the above technique, however, the metal layers are simply broughtinto contact with each other through thermal pressurization andcompression. Accordingly, the above technique is problematic in thatsufficient water-resistance cannot be obtained because the coupling ofthe metal layers is not robust and the metal layers are separated fromeach other because the coupling of the metal layers is weakened becauseof a long-term use.

Japanese Unexamined Patent Application Publication No. 2004-055154discloses a technique for preventing the leakage of an electrolyte andthe penetration of moisture resulting from the exposure of an innerresin layer by extending the peripheral portion of one of a pair oflaminate films more outward than the peripheral portion of the other ofthe pair of laminate films and performing laser welding on a portionwith which the front end of the extended portion, bent toward theperipheral portion of one of the pair of laminate films, is brought intocontact so that the outermost layer is volatilized by the heat of thelaser light and the ends of both metal layers are melted and combinedtogether.

However, the above method is problematic in that it requires a highdegree of accuracy in terms of the process because one of the pair oflaminate films must be accurately bent enough to seal the other of thepair of laminate films and the cell manufacture costs are increasedbecause the process is added.

Further, Japanese Unexamined Patent Application Publication No.2000-223090 discloses a technique in which a metal layer and a thermalcompression layer are stacked, part of a thermally compressed portiontoward the inside of a cell casing is removed to expose the metal layer,and dual sealing processing, including the thermal compression of thethermal compression layer and the welding of the metal layer, isperformed when a cell is sealed.

However, the above method is problematic in that the process iscomplicated and the costs are increased because the dual sealingprocessing is performed.

SUMMARY OF THE INVENTION

Accordingly, the present invention has been made keeping in mind theabove problems occurring in the prior art.

The inventors of this application have found that after variousexperiments and deep researches, if steps are formed at the tips of theouter sides of sealing units, a phenomenon in which materials within acell are eroded by a side reaction within the cell resulting from thepenetration of moisture can be fundamentally prevented and so the lifespan and stability of the cell can be improved.

In accordance with an aspect of the present invention, there is provideda pouch type secondary cell, comprising a positive electrode, aseparation layer, and a negative electrode. Here, the sealing unit ofthe secondary cell includes steps so that the sealing unit has an outerside thinner than an inner side in thickness.

In the pouch type secondary cell of the present invention, each of thesteps comprises one or more steps discontinuously formed from the innerside to the outer side. In the pouch type secondary cell of the presentinvention, each of the steps is continuously formed from the inner sideto the outer side.

In the pouch type secondary cell of the present invention, a thicknessof an outermost internal resin layer of the sealing unit is 3 to 50% ofa thickness of the internal resin layer not thermally compressed.

In the pouch type secondary cell of the present invention, a thicknessof an innermost internal resin layer of the sealing unit is 50 to 95% ofa thickness of the internal resin layer not thermally compressed.

In the pouch type secondary cell of the present invention, the sealingunit has a width ranging from 3 to 20 mm.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention willbecome apparent from the following description of preferred embodimentsgiven in conjunction with the accompanying drawings, in which:

FIGS. 1 and 2 are perspective views showing the structure of aconventional pouch type secondary cell;

FIG. 3 is a diagram showing a process of forming the sealing unit of alaminate sheet which is commonly used in a pouch type secondary cell;

FIGS. 4 to 6 are perspective views an embodiment of the presentinvention; and

FIG. 7 is a graph showing a comparison of the time and the amount ofgenerated hydrogen fluoride (HF) in the case in which steps are formedin sealing units and in the case in which the steps are not formed inthe sealing units.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Hereinafter, some embodiments of the present invention will be describedin detail with reference to the accompanying drawings so that it can bereadily implemented by those skilled in the art.

The present invention relates to a pouch type secondary cell, includinga positive electrode, a separation layer, and a negative electrode,wherein the sealing unit of the secondary cell includes steps so thatthe sealing unit has an outer side thinner than an inner side inthickness.

In other words, in accordance with the present invention, in the pouchtype secondary cell manufactured by receiving the electrode assembly ofthe positive electrode, the separation layer, and the negative electrodein a pouch type cell casing and sealing the cell casing using thermalcompression, sealing units sealed by the thermal compression sealing hasan outer side which is thinner than an inner side in thickness.

The term “outer side” refers to a place or portion relatively far fromthe reception unit of the electrode assembly placed at the center of thecell casing, and the term “inner side” refers to a place or portionrelatively close to the reception unit of the electrode assembly placedat the center of the cell casing.

As can be seen from Equation 1 below, the amount of moisture penetratedthrough the sealing units is proportional to the cross section (L*T) ofan internal resin layer which brings into contact with the outside. Inthe present invention, the steps are formed in the sealing units, andthe outer side of the step is thinner than the inner side thereof in thethickness of the internal resin layer in order to enhancewater-resistance.

Mass transfer rate=D*□C/W*L*T   Equation 1

where D is a diffusion coefficient, □C is a difference in theconcentration, W is a position, L is the width of the cross section ofthe internal resin layer, and T is the height (thickness) of the crosssection of the internal resin layer.

That is, in the present invention, the steps allow the internal resinlayer of a laminate sheet to be externally exposed to the minimumextent, thereby enhancing the water-resistance and the sealing propertyof the cell. The internal resin layer is exposed at a portion where thelaminate sheets are combined together. The exposed internal resin layeris chiefly made of polymer resin, and so moisture can easily penetrateinto the exposed internal resin layer. The penetrated moisture generatesbad problems in terms of cell safety, such as that oxidizes materialswithin the cell and the metal layer of the cell casing, weakens theadhesion strength of the sealing units, and possibly leaks anelectrolyte and also causes to reduce the life span and stability of thecell when the cell is used for a long time. In the case in which LiPF₆lithium salt is included in the electrolyte, LiPF₆ must exist in theform of ions of Li⁺ and PF₆ ⁻. However, unstable PF₅ is created asbyproducts because of an unwanted side reaction. The created unstablePF₅ reacts to H₂O, thereby forming hydrogen fluoride (HF). The formed HFbreaks a SEI layer and causes to dissolve the positive electrode. Thisphenomenon is more remarkable at high temperature.

Accordingly, the internal resin layer can be formed to have a thinthickness in order to enhance water-resistance. However, to form theinternal resin layer having a thin thickness unconditionally is notpreferred. With a reduction in the thickness of the internal resinlayer, the internal electrolyte is likely to leak externally and acontact can occur between the metal layers or between the metal layerand the electrolyte. Accordingly, the insulating property of the cell ismuch deteriorated.

Accordingly, in the present invention, to solve the above problems, thesteps each having the inner side and the outer side with a differentthickness are formed. In other words, the outer side of the step has arelatively thin thickness in order to enhance water-resistance, and theinner side of the step has a relatively thick thickness in order toguarantee the insulating property and the sealing property of the cell.In the present invention, the step can be formed in various ways.

The outer side of the sealing unit can be formed under a relativelyhigher sealing pressure than the inner side of the sealing unit. Inaccordance with an embodiment of the present invention, a method offorming the laminate sheet through thermal compression using a rollerhaving a step formed therein or a method of forming the laminate sheetthrough thermal compression using a bar having a step formed therein canbe used (that is, the step can be easily formed simultaneously with thesealing of the cell casing by thermally compressing the laminate sheetusing the roller or bar having the same shape as a step to be formed).

As shown in FIGS. 4 to 6, the step of the cell casing may be formed tohave one discontinuous step from the inner side to the outer side, butmay be formed to have one or more discontinuous steps. Alternatively,the step may be continuously formed from the inner side to the outerside.

In the present invention, the step has the inner side thinner than theouter side in thickness. Accordingly, a difference between the high andthe low has only to exist as shown in FIGS. 4 to 6, the shape of thestep is not limited.

In the present invention, the thickness of the sealing unit pertinent tothe step is not specially limited, and the thickness of the outermostinternal resin layer of the sealing unit preferably is 3 to 50% of thethickness of the internal resin layer that has not been thermallycompressed. If the thickness of the outermost internal resin layer ofthe sealing unit exceeds 50% of the thickness of the internal resinlayer that has not been thermally compressed, water-resistance becomespoor. If the thickness of the outermost internal resin layer of thesealing unit is less than 3% of the thickness of the internal resinlayer that has not been thermally compressed, the insulating property ofa cell may not be good because the metal layers of the laminate sheetare brought into contact with each other.

Further, the thickness of the innermost internal resin layer of thesealing unit preferably is 50 to 95% of the thickness of the internalresin layer that has not been thermally compressed. If the thickness ofthe innermost internal resin layer of the sealing unit is less than 50%of the thickness of the internal resin layer that has not been thermallycompressed, it is difficult to prevent the leakage of an electrolyte andthe insulating property of a cell is poor. If the thickness of theinnermost internal resin layer of the sealing unit exceeds 95% of thethickness of the internal resin layer that has not been thermallycompressed, the sealing property can be deteriorated.

In general, the internal resin layer before the laminate sheet used asthe cell casing is subject to thermal compression has a thickness of 0.5to 50 μm.

The width of the sealing unit is not specially limited, but preferablyin the range of 3 to 20 mm.

Further, the present invention provides a method of manufacturing thepouch type secondary cell, including the steps of forming the electrodeassembly by alternately and sequentially stacking the positiveelectrode, the separation layer, and the negative electrode; receivingthe electrode assembly in the pouch type cell casing; and forming thesealing units in the cell casing by thermally compressing the cellcasing. Here, when the cell casing is thermally compressed, the stepsare formed in the respective sealing units having the outer side thinnerthan the inner side in thickness.

In the method of manufacturing the pouch type secondary cell accordingto the present invention, the sealing unit can be formed using a methodof thermally compressing the laminate sheet using a roller having a stepformed therein or a method of thermally compressing the laminate sheetusing a bar having a step formed therein (that is, the step can beeasily formed simultaneously with the sealing of the cell casing bythermally compressing the laminate sheet using the roller or bar havingthe same shape as a step to be formed). Further, the outer side of thesealing unit can be sealed and formed using a relatively higher sealingpressure than the inner side of the sealing unit.

Hereinafter, an embodiment of the present invention is further describedwith reference to the accompanying drawings, but the scope of thepresent invention is not limited thereto.

FIG. 4 is a diagram showing the pouch type secondary cell in which thesteps are formed in the sealing units according to an embodiment of thepresent invention.

Referring to FIG. 4, the pouch type secondary cell 100 a has a structurein which electrode leads 411 a and 421 a are symmetrically formed on theupper and lower sides of a cell casing 201 a, respectively. The pouchtype secondary cell 100 a is manufactured through thermal compressionusing a roller having a step formed therein in the state in which anupper casing 231 a and a lower casing 221 a separated from each otherare embedded in an electrode assembly (not shown). Here, the steps areformed at the tips of the sealing units. The steps 600 and 700 areformed up and down in upper and lower sealing units 241 a and 261 a andsealing units 251 a and 271 a on both sides of the cell casing which areformed through thermal compression.

When the steps are formed at the tips of the sealing units as describedabove, the area in which the internal resin layer 13 is exposedexternally can be reduced. Further, part of the internal resin layer 13not exposed externally can be formed thickly.

FIGS. 5 and 6 are diagrams showing the shapes of steps according toanother embodiment of the present invention. In the embodiments of thepresent invention, the steps can include two or more discontinuoussteps, as shown in FIG. 5, or can include numerous continuous steps, asshown in FIG. 6.

From FIG. 7, it can be seen that in the present invention in which thesteps are formed in the sealing units, water-resistance is enhanced andthe amount of hydrogen fluoride (HF) generated within the cell accordingto a lapse of the time is further reduced, as compared with a case inwhich the steps are not formed in the sealing units.

As described above, the present invention is advantageous in that themanufacture process can be simplified, the water-resistance and sealingproperty of sealing portions can be further improved, and themanufacture costs can be reduced.

While the invention has been shown and described with respect to thepreferred embodiments, it will be understood by those skilled in the artthat various changes and modifications may be made without departingfrom the spirit and scope of the invention as defined in the followingclaims.

1. A pouch type secondary cell, comprising a positive electrode, aseparation layer, and a negative electrode, wherein sealing unit of thesecondary cell includes steps so that the sealing unit has an outer sidethinner than an inner side in thickness.
 2. The pouch type secondarycell of claim 1, wherein each of the steps comprises one or more stepsdiscontinuously formed from the inner side to the outer side.
 3. Thepouch type secondary cell of claim 1, wherein each of the steps iscontinuously formed from the inner side to the outer side.
 4. The pouchtype secondary cell of claim 1, wherein a thickness of an outermostinternal resin layer of the sealing unit is 3 to 50% of a thickness ofthe internal resin layer not thermally compressed.
 5. The pouch typesecondary cell of claim 1, wherein a thickness of an innermost internalresin layer of the sealing unit is 50 to 95% of a thickness of theinternal resin layer not thermally compressed.
 6. The pouch typesecondary cell of claim 1, wherein the sealing unit has a width rangingfrom 3 to 20 mm.